In Reply: Dr Beasley notes that our title implied
demonstration of causation whereas, in fact, we found an association that
may or may not indicate causation. He is correct and we regret our linguistic
imprecision. Beasley further suggests that past research may have shown fitness
to be a better predictor of mortality rate (MR) than BMI. He also suggests
that there are potential causal interconnections among BMI, sedentariness,
dietary intake, physiological variables, and MR. We are not certain that fitness
has been shown to be a better predictor of MR than BMI or obesity, nor are
we clear what it means to be a "better" predictor. A more critical question,
in our view, is whether each predictor has independent causative influences
on MR. Some data do suggest that BMI, fitness, and activity levels all have
independent associations with MR.1 If independent
associations of these variables imply independent effects on MR, then the
fact that one of them is a predictor in no way obviates the importance of
the others. Clearly more research will be needed to elucidate the complex
and interactive relations among body weight, body composition, fitness and
physical activity levels, physiological variables, and MR. As Fisher wrote,
"In no case, however, can one judge whether or not it is profitable to eliminate
a certain variate unless we know, or are willing to assume, a qualitative
scheme of causation."2